Cathode-ray indicator system



April 26, 1949. H. w. LEVERENZ ATHODE-RAY INDICATOR SYSTEM 2Sheets-Sheet 1 Filed March 29, 1946 .l I up.

F Wn rv BY CM April 26, 1949. H. w. LEVERENZ CATHODE-RAY INDICATORSYSTEM Filed MaCh 29,A 1946 2 Sheets-Sheet 2 70 offz fer/0N ry/VCH.

INVENToR. IU/IIa/a W Lef/@P61212 TTRVEX isatented pr. 2,6,

CATHODE-RAY INDICATOR SYSTEM Humboldt W. Leverenz, Princeton, N. J.,assignot to Radio Corporation of America, a corporation of DelawareApplication March 29, 1946, Serial No. 657,972

(Cl. Z50-164) 12 Claims. 1

This invention relates to cathode ray indicator systems, and moreparticularly to improvements in systems or the type in which the imageto be exhibited is traced out during an extended scanning period, Whenthe scanning period is of the order of one-tenth second or less, allparts of the image appear visible simultaneously owing to persistence ofvision. With longer scanning periods, special long-persistence screensare employed.

The materials used in the screen, and their proportions, are selected toprovide as nearly as possible the best compromise between maximumluminescence of the image during the scanning period and minimumcarry-over of an image into the following scanning period. Principallyas a result of the remarkable tolerence of the human eye to variationsin brightness, such compromises can be effected with reasonable success.However, a different screen material, or phosphor, is required foroptimum results with each diierent length of scanning period. In manyapplications of cathode ray tubes, such as oscillography and radarindication, it is desirable to use scanning periods of variable ordifferent lengths.

The principal object of the present invention is to provide improvedmethods of and means for cathode ray indication which aiiord images ofsubstantially constant luminance.

Another object of the present invention is to provide improved methodsof and means for cathode ray indication which afford images whoseluminance and persistence can be varied at will.

Another object of this invention is to provide improved methods of andmeans for cathode ray indication which exhibit negligible carry-over ofan image from one scanning period to the next.

A further object is to provide, in systems of the described type,methods of and means for achieving the foregoing objects substantiallyindependently of the scanning period durations, and of the normalconcave upward phosphorescence of luminescent materials.

Another object is to provide methods of and means for efficientutilization of substantially all of the available excitation energy in aluminescent indicator system.

The invention will be described with reference to the accompanyingdrawing, wherein:

Figure 1 is a schematic diagram of a cathode ray indicator systemembodying the present invention,

Figure 2 is a graph showing the characteristics of an infrared sensitivephosphor used in the system of Figure 1,

Figures 3, 4 and 5 are graphs illustrating variations of the stimulationof a phosphor, as practiced in accordance with the present invention,and the response of said phosphor thereto, and

Figure 6 is a modied cathode ray indicator system according to theinvention.

Referring to Figure 1, two substantially identical cathode ray tubes land 3 are disposed at right angles to each other, with a sheet 5 ofplate glass or similar material between them at 45 degrees. The tubes iand 5 are provided with deiiecting yokes 'l and S respectively,connected to a deflection generator I l.

The beam intensity control electrode o1" the tube i is connected througha high resistance I3 to a biasA source l5. The intensity controlelectrode or" the tube 3 is similarly connected through a resistor il toa bias source I9. A double throw switch 2l connects either the intensitycontrol electrode of the tube l or that of the tube 3 to a source, notshown, of signals corresponding to the images to be displayed. rIheinput signals may be variably timed pulses, such as are obtained in theoperation of certain known radio locator or radar systems, or a variablevoltage, corresponding at each instant to the intensity of a respectiveelement of the image.

The screens of the tubes l and 3 are coated with a stimulable phosphor,which has the ability to store energy supplied to it directly orindirectly by the cathode ray beam, and release energy in the form ofvisible light when stimulated. One example of such a phosphor isstrontium sulphide containing small amounts of samarium and cerium. Thismaterial will absorb and store cathode ray energy, and give up a portionof the stored energy as visible light when irradiated by infrared.

Another type of stimulable phosphor is described in copending U. S.Patent Application Serial No. 595,146, led May 22, 1945 by H. W.Leverenz, and entitled Phosphor material. This phosphor has thecharacteristic of being excited or sensitized by blue, green, orultraviolet light, and stimulated by infrared or heat.

Figure 2 shows the light output as a function of time, in a typicalcycle of excitation, stimulation, and quenching. During the period To toT1, the phosphor is being excited by blue or ultraviolet light ofconstant intensity. Some of the energy of the exciting light isconverted immediately to visible light of longer wavelength, byuorescence. This effect builds up at a decreasing rate as shown by theportion 23 of the graph. If excitation were continued indenitely, thefluorescence would nally reach a constant Value corresponding to thedash line 25 marked Saturation The area above the curve, bounded by thelines 23, 25, To and T1, represents energy stored in the phosphor, whilethe area under the curve 23 repre.- sents energy released immediately aslight. When the exciting energy is cut oli at T1, the fluorescencesubsidesquickly as shown by the line 2l. The stored energy remains inthe phosphor, and if not liberated by stimulation or quenching, willremain to a substantial extent for a period of days.

During the period T2 to T3, the phosphor is being stimulated byirradiationr with infrared of constant intensity. The solidV line 29.shows thelight output with one intensity of infrared, while the dashline curve 3l shows the light. output which would result with a higherintensity of in-y frared. The color of the light emitted. understimulation is generally the same as that produced by uorescence in theexcitation period To-T1.

When stimulation is stopped at the time T3, the phosphor stops emittinglight. Unless substantially all of the stored energy has been used,

remainder of the stored energy. The cycle of excitation, stimulation andquenching can then be repeated. The above mentioned strontium sulphidephosphor responds to stimulation and quenching substantially like thatdescribed with reference to Figure 2, but is excited directly by jcorpuscular radiations, such as cathode rays or ionic rays, instead ofby blue or ultraviolet light.

Either a directly or an indirectly excited phosphor may be used in thepractice of the present invention. If the latter is employed, thescreens of the tubes i and 3 are provided with two layers: an innerlayer, made of any of the known phosphors, which provide blue, green, orultraviolet light in response to cathode rays, and an outer layer ofstimulable material, which may be either insideor outside the glassenvelope ofy the cathode ray tube, and is excitable by emission from theinner layer.

Resuming the description of Figure 1, a pair ofl infra-red lamps 33. and35 are positioned to i3A ood the screens of the tubes l and 3respectively. The lamps 33 and 35 are connected to their power source3'! through a resistor network described hereinafter, and a double throwswitch 39. The

switch 33 is mechanically coupled to the switch 7:'

21v so that when the input signal is applied to one cathode ray tube,inflared light is applied to the other.

The switches 2i yand 39 are operated cyclically from one position to theother by a cam M, driven by a motor Q3. The motor 43 also drives a; cam45, operating a switch W to periodically connect a D.C. source 49 to thedeiiection generator H. This provides a synchronizing signal to causethe deection generator to initiate a scanning'pattern on the cathode raytubes l and 3; Wit-h each operation oi the switches 2l and 39. Arheostat 5i is connected between the motor 43 and its power supply 53for adjustment of the motor speed and hence the scanning repetitionfrequency.

Ayariable resistor device 55 is provided in the supply circuit to theinared lamps 33 and 35. The device 55 includes a rotatable contact 51,coupled to the motor 43 and connected to the switch 39. rIwo arcuateresistor elements 59 and 5l are provided, with the upper end of one andthe lower end of the other connected together and to the power supply.As the contact 5l is rotated clockwise by the motor 53, the resistanceintroduced by the device 55 is relatively high at each instant theswitch 39 operates, and decreases until the switch 39 operates again. Arheostat 53, like the rheostat 5l, is connected between the device 55and the source 3l'. The rheostats 5| and 53 may be mechanically gangedfor common operation.

In thel operation of the system of Figure l, the rheostat 5l is adjustedto make the motor 43 run at a speed corresponding to one-half thedesired image repetition rate. Thus, if a complete image is. scannedevery ten seconds, the motor runs at three revolutions per minute.Suppose the switches 2l and 33 are initially in their upper positions.The input signal is then applied to the tube 3, and the image is tracedout by the cathode ray on the screen `of that tube, storing an identicalimage in the infrared sensitive phosphor. The cathode ray beam or thetube l is meanwhile cut cil by the bias supplied by the source I5.

During the next scanning period, the switches 2l and are in their lowerpositions. The input is applied to the tube l and the current image isstored in its screen. While this is taking place, the screen of the tube3 is ooded with infrared from the lamp 35. The stored image is luminousin response to the infrared excitation, and may be viewed through thetransparent plate 5 from the point E. During the Viewing period, whichis preferably (though not necessarily) of the same length as thescanning period, the energy stored in the screen of the tube 3 is beingreleased as light.

If the infrared excitation were maintained constant, the luminance ofthe image would decay in a manner like that indicated by the line 23 orthe line 3l in Figure 2. However, the variable resistance device 55operates as described abovel to decrease the resistance and thusincrease the current to the infrared lamp. IThe device 55 is designed sothat the increase in excitation compensates the decrease in remainingavailable stored energy, so that each element of the image beingdisplayed has a substantially constant luminance during the viewingperiod.

Referring to Figure 3, the dotted curve 55 shows the luminance as afunction of time which would appear with constant infrared excitation.The solid line ill indicates the intensity of infrared as it is variedby the device 55. The dash line curve shows the resulting stimulatedluminance. The resistor 63 is adjusted so'that the total infrared energyapplied to the screen of the cathodel ray tube during the viewing periodis just suil'icient to substantially exhaust the energy stored thereinduring the previous scanning cycle. Thus, at the end of the viewingperiod, the image decays rapidly and completely, leaving the screenprepared for storing another image. Moreover, substantially all of theavailable excitation energy is utilized to provide luminescence duringthe viewing period, instead of carrying over into the subsequent scan asin prior art systems. Figure 4 shows the conditions with a viewingperiod shorter than that of Figure 1. The rheostat 63 is adjusted toprovide a higher intensity of infrared, so that the total energyintegrated throughout the viewing period is again substantially enoughto exhaust the stored energy. As bef-ore, the intensity increases duringthe periodito'maintain a substantially constant rate of light output.

The above described cycle is repeated with each scanning cycle, first inone and then in the other of the tubes l and 3. The images on the tube lare reflected by the plate 5 and viewed from the same position E asthose on the tube 3.

It will be apparent that the stimulation may be varied to providesubstantially any desired variation in luminance of the stimulatedimage. For example, if the infrared intensity is increased at a rategreater than that corresponding to the rate of decay of the storedenergy, as shown by the line 61 in Figure 5, the stimulated luminancewill actually increase, as shown by the dash line curve '69 in Figure 5.

Owing to fluorescence during the excitation periods, the image beingstored will be visible at the point where the cathode ray beam strikesthe screen. Although this may be prevented from reaching the observer bymeans of a shutter which is closed while the screen is scanned, it maybe advantageous in affording direct visual comparison between successiveimages, as described in copending U. S. patent application Serial No.657,971 filed on March 29, 1946, by H. W. Leverenz, and entitled Radarsystem.

It will be apparent. to those skilled in the art that numerousmodications of the system of Figure l may be made without departing fromthe spirit of the present invention. One example is illustrated inFigure 6, wherein a single cathode ray tube ll is required. The tube llis provided with a deflection yoke connected to a source of deflectioncurrent like the generator Il (not shown in Figure 6). The input isapplied directly to the intensity control electrode, so that eachsuccessive image is traced on the screen of the tube ll. The screen inthis case is coated with a blue, green, or ultraviolet phosphor and neednot include any stimulable material.

A transparent disc '13, coated with stimulable phosphor, is supportedoff-center in front of the screen of the tube "H, on a shaft 15. Theshaft 75 is coupled through gears ll to a ratchet wheel '19, engaged bya pawl 8l on a reciprocable rod 83. The lower end of the rod 83 engagesa spiral cam B5, driven by the motor 43. The rod 83 is provided withrack teeth B1 engaged by a pinion B9 on a shaft 9i. A rheostat 93 iscoupled to the shaft 9 l An infrared lamp 95 is connected to a powersource 91 through the rheostat 93, and is directed to irradiate an area99 on the disc '3. An orangeemitting lamp IBI is connected to the source91, and is directed to illuminate an area |63 on the disc 13.

The adjustment and operation of the system of Figure 6 is as follows:

The motor 43 runs at a speed (set by the rheostat 5l) of one revolutionper scanning cycle. The cam 35 is set so that the rod 83 is at itslowest position at the beginning of each scanning cycle. The imagetraced on the screen of the cathode ray tube 'Il is stored, as itappears, on the area |95 on the disc 13. Meanwhile the cam 85 lifts therod 83, which reaches its uppermost position substantially at the end ofthe scanning cycle. At this time, the rod 83 moves quickly to its lowerposition. The pawl 8| rotates the ratchet wheel 'I9 through a fractionof a turn. The gears l1 are proportioned so that the fractional turn ofthe wheel 'I9 rotates the disc 73 through 120 degrees, in the presentexample.

At any given time, the image stored during the last previous scanningcycle is at the position 99. At the beginning of the viewing period, therod 83 is in its lowermost position and the pinion 89 is at its extremeclockwise position, so that the rheostat 93 is set to provide itsmaximum resistance. As the viewing period progresses, the rod 83 and itsrack 81 are raised by the cam 85, rotating the shaft 9icounter-clockwise to reduce the resistance of the rheostat 93. Thisincreases the intensity of the infrared light from the source 95,producing substantially constant luminance of the image stimulatedthereby in the area 99 of the disc 73.

Although the rheostat 63 may be adjusted as in the system of Figure 1,to provide substantial exhaustion of the stored image during the Viewingperiod, there may be a certain amount of residual stored energy undersome conditions of operation. After each viewing period, the last viewedarea of the disc 'I3 is flooded, at the position |93, with orange light.This quenches or dissipates any vestiges of the stored image inpreparation for the next exposure to the cathode ray tube Tl.

summarizing briefly, the present invention provides controllableluminance of a cathode ray image during a viewing period whose length isnot restricted to a denite period, as in prior art practice, by thecharacteristics of the particular phosphor used. The cathode ray imageis stored as it is traced out, in a screen of stimulable phosphor, andsubsequently rendered visible as a whole by stimulating the phosphor. Tomaintain constant luminance during the viewing period, the intensity ofstimulation is increased as the available stored energy decreases. Toprovide a maximum luminous efficiency, the average level of stimulationmay be set to provide substantial exhaustion of the stored energy duringthe viewing period.

I claim as my invention:

l. A visual display system of the type in which an image is built up bysuccessive presentation of its elements during a scanning period,including a screen coated with an infrared stimulable phosphor, meansfor applying said image elements to said phosphor as presented to storetherein a latent image, a source of infrared energy, means forirradiating said phosphor with a predetermined quantity of infraredenergy from said source to render said latent image visible during aviewing period subsequent to said scanning period, said quantity beingsubstantially that required to exhaust said latent image, and means forincreasing the intensity of said infrared irradiation during saidviewing period.

2. A visual display system of the type in which an image is built up bysuccessive presentation of its elements during a scanning period,including a screen coated with an infrared stimulable phosphor, meansfor applying said image elements to said phosphor as presented to storetherein a latent image, a source of infrared energy, means forirradiating said phosphor with a predetermined quantity of infraredenergy from said source to render said latent image visible during aviewing period subsequent to said scanning period, said quantity beingsubstantially that required to exhaust said latent image, and means forincreasing the intensity of said infrared irradiation during saidviewing period, at a rate cory responding to the rate of decay of saidstored image.

3. A visual display system of the type in which an image is built up bysuccessive presentation of its elements during a scanning period,including a screen coated with an infrared stimulable phosphor, meansfor applying said image elements to.I said phosphor as presen-ted tostore therein a latent image, a source of: infrared energy, means forirradiating said phosphor with infraredV energy from said source torender said latent image visible during a viewing period subsequent tosaid scanning period, and means for increasing the intensity of saidinfrared irradiation during said viewing period.

4. A cathode ray indicator system of the type in which an image istraced out during an extended scanning period, including a screen coatedwith a stimulable phosphor, means for tracing said image with excitingenergy on said screen during said scanning period to store said image inlatent form in said phosphor, a source of stimulating energy, means forapplying said stimulating energy to said phosphor during a viewingperiod subsequent to said scanning period to render ,said latent imagevisible, and means for increasing the intensity of said stimulatingenergy during said Viewing period to maintain substantially constantluminance of said Visible image.

5. A cathode ray indicator system of the type in which an image istraced out during an extended scanning period, including a screen coatedwith -a stimulable phosphor, means for tracing said image With excitingenergy on said screen during said scanning period to store said image inlatent form in said phosphor, a source of stimulating energy, means forapplying a predetermined quantity of said stimulating energy to saidphosphor during a viewing period subsequent to said scanning period torender said latent image visible, said quantity being substantially thatrequired to exhaust said latent image Within said viewing period, andmeans for increasing the intensity of said stimulating energy duringsaid viewing period to maintain substantially constant luminance of saidvisible image.

6. In a visual display system wherein an image is built up during anextended scanning period by sequential presentation of its elements, themethod of exhibiting said image as a Whole during an extended viewingperiod, comprising the steps of storing said image as presented in abody of s timulable phosphor, applying stimulating energy yto said bodyto render said image visible during said viewing period, and increasingthe intensity o f said stimulating energy during said view-ing period.

7. 'In a visual display system wherein an image is built up during anextended scanning period by sequential presentation of its elements, themethod of exhibiting said image as a Whole and with substantiallyconstant luminance during an extended viewing period, comprising thesteps of storing said image as presented in a body of stimulablephosphor, applying a predetermined quantity of stimulating energy tosaid body to render said image visible during said Viewing period, saidquantity being substantially that required to exhaust said stored image,and increasing the intensity of said stimulating energy during said 8viewing period at a rate eorrfenondine to the' rate of decay of saidstored image.

8. lIn a cathode ray indicator system of the type in which an image istraced out during an .extended scanning period, the method ofvexhibiting said image with substantially constant luminance during anextended Viewing period, cornprising the steps of `storing said image ina stimulable phosphor during said scanning period, applying stimulatingenergy to said phosphor dur- .ing said Viewing period, and increasingthe intensity of said stimulating energy during lsaid Viewing period ata rate corresponding to the rate of decay oi said stored image.

9. in a cathode ray indicator ,systernof the type in which an image istraced out during an vextended scanning period, the method ,ofexhibiting said image with substantially constant luminance during anextended Viewing period, ,comprising the steps of storing said Iimage ina stimulable phsphor during said scanning period, applying stimulatingenergy to said phosphor during said viewing period, and increasing theintensity of said stimulating energy during said viewing period at 4aratecorresppnding to the rate of decay of said stored image, the averageValue of said rintensity being such that said stored image issubstantially exhausted at the end of Viewing period. t

l0. The method of exhibiting a luminous image with substantiallyconstant luminance duringa viewing period of predetermined length,comprising the steps of storing the image to b ,e exhibited in a body ofinfrared stimulable phosphor, Vapplying infrared energy to said phosphorto render said stored image visible during said Viewing period, andncreasing the intensity of said infrared energy during said period tocompensate the decrease in available energy of said stored image.

11. AThe method of exhibiting a luminous image with controllableluminance 4vduring a Viewing period of predetermined length, comprisingthe steps of storingfthe image to be exhibited ina body of stimulablephosphor, stimulating said phosphor to render said stored image Visibleduring said viewing period, and increasing the intensity of stimulationduring said period.

l2. IThe invention set forth in claim 11, wherein the average intensityoisaid stimulationis vsuch as to substantially exhaust the venergyofdsaid stored image at the end of said viewing period.

HUMBOLDT W. LEVERENZ.

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